Technical Field
The present disclosure relates to a method for controlling an exhaust after-treatment apparatus for a vehicle and more particularly, a method for controlling an exhaust after-treatment apparatus for a vehicle that improves exhaust after-treatment performance by controlling an amount of ammonia (NH3) generated by a Lean NOx Trap (NLT).
Background Art
With the strengthening of vehicle emission regulations, a DeNOx catalyst technique (e.g., Lean NOx Trap (LNT), Selective Catalytic Reduction (SCR) and the like) has been applied to an after-treatment apparatus to reduce nitrogen oxides (NOx) in exhaust gas.
The DeNOx catalyst is a type of catalyst converter that removes NOx included in exhaust gas. The DeNOx catalyst causes an oxidation-reduction reaction between NOx and a reducing agent (e.g., urea, ammonia (NH3), carbon monoxide (CO), or hydrocarbon (HC)), to reduce NOx by the oxidation-reduction reaction with the reducing agent.
Recently, a LNT (or referred to as a LNT catalyst) has been used as an after-treatment apparatus to remove NOx from exhaust gas ingredients generated when a lean-burn engine operates. The LNT absorbs or occludes NOx included in exhaust gas in a lean environment, and desorbs the absorbed or occluded NOx in a rich environment.
A SCR system may effectively reduce NOx by supplying a reducing agent to a SCR catalyst. The SCR system supplies a reducing agent to exhaust gas to reduce NOx, unlike an Exhaust Gas Recirculation (EGR) apparatus of reducing NOx by recirculating exhaust gas to lower the combustion temperature of a combustion chamber. “Selective Catalyst Reduction (SCR)” means making a reducing agent, such as urea, NH3, CO, HC, and the like, react with NOx among oxygen and NOx.
A Diesel Oxidation Catalyst (DOC), a Diesel Particulate Filter (DPF), and a Catalyzed Particulate Filter (CPF) have been developed and used within vehicles to reduce particulates from exhaust gas. Recently, a SCR on Diesel Particulate Filter (SDPF) that collects particulates and reduces NOx has been used.
The SDPF, which is manufactured by coating a porous DPF with a SCR catalyst, causes NH3 to react with NOx in exhaust gas within the SCR catalyst to generate water and nitrogen (N2), while collecting particulates in the exhaust gas though the filter function, that is, the DPF function. Accordingly, although various after-treatment apparatuses are used to meet vehicle emission regulation, strengthening of the vehicle emission regulations requires an after-treatment apparatus with greater optimal performance. Meanwhile, in the LNT catalyst, a NOx absorbing catalyst and a Diesel Oxidation Catalyst (DOC) are included within a carrier. When the engine is driven in a lean mode, NOx is absorbed by a catalyst washcoat, and when the engine is driven in a rich mode, diesel fuel is used as a reducing agent to reduce the absorbed NOx to nitrogen (N2).
Generally, a diesel engine is driven in a lean mode, in which an amount of air that enters the engine is more than that of an equivalence ratio, and NOx generated when the diesel engine is driven in the lean mode is absorbed within a LNT catalyst, which is a NOx Storage Catalyst (NSC). To reduce the NOx absorbed in the LNT catalyst to nitrogen (N2), a throttle valve is closed by a predetermined amount to reduce inflowing air, and post combustion is induced to switch the lean mode to the rich mode.
For driving in the lean mode and the rich mode, signals from lambda sensors or NOx sensors installed before and after the LNT catalyst are used. However, since NOx sensors are expensive, lambda sensors are generally used. When NOx absorbed within the LNT catalyst reaches a predetermined level, the lean mode is switched to the rich mode to commence NOx regeneration control from a predetermined level (e.g., a level ranging from about 0.92 to about 0.94) based on a signal from the lambda sensor installed before the LNT catalyst, and a reducing agent generated by driving in the rich mode, acts to reduce NOx absorbed within the LNT catalyst to N2.
In the LNT catalyst, the amount of the absorbed NOx is gradually reduced, and as the rich mode continues while reactants decrease, an amount of slipped reducing agents increases. Accordingly, a value detected by the lambda sensor installed after the LNT catalyst gradually converges to a value detected by the lambda sensor installed before the LNT catalyst, which represents that reducing agents are slipped after the LNT catalyst.
The above information disclosed in this section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.